This invention relates to cold trap apparatus for the purification of liquid metals such as sodium.
It is known, in the field of sodium-cooled nuclear reactors, to lower the temperature of a fraction of a circulating coolant in a vessel externally cooled and having a meshwork filling so that impurities in the sodium (such as oxides and hydrides) are initially retained in the cooled sodium in a supersaturated solution form and, on passing through the meshwork filling, are precipitated by crystallisation on to the meshwork. Such devices are known as "cold traps" and flows take place therethrough at a low velocity over large surface areas. Traditionally the meshwork is in the form of mesh-filled basket units which offer said large surface area.
In the use of known traps a number of features have now been noted. For example, the deposition of impurities tends to be predominantly at the inlet faces of the units, the deposition takes place in a non-uniform manner, and deposition can occur on the walls of the cooled vessel. Additionally sudden onset of blocking can occur with loss of purification. These features are not conducive to cost advantageous design and operation and the present invention seeks an improvement in relation to them.